<div class="container">
    <span class="text">
        I'm one line of text!
    </span>
</div>
html {
    padding: 20px;
}

.container {
    height: 40px;
    background: #eee;
}
.container > .text {
    display: block;
    line-height: 40px;    // Equal to .container's height
    text-align: center;
}
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